There is currently a need for antennas and/or antenna systems that can communicate with both a satellite and a terrestrial system. One example of such a need is for a Direct Broadcast Satellite (DBS) radio in which radio signals are broadcasted from a satellite and are received by a receiver located on the vehicle and are also received by terrestrial repeaters which rebroadcast the signals therefrom to the same vehicle. Typically, a DBS uses circular polarization so the vehicle can receive the transmission in any orientation. However, terrestrial networks typically transmit in linear, vertical polarization. If satellite communication fails (e.g., if the satellite becomes hidden by a building or by another object, man-made or natural), then the terrestrially rebroadcast signal can be used to fill in the gaps in the satellite signal.
DBS radio systems typically have a narrow bandwidth (about 0.5%) due to the low power available from satellites, as well as the problems associated with mobile wireless communications.
On the other hand, an antenna is typically designed with at least several percent bandwidth to account for possible errors in manufacturing. For this reason, the antennas used to receive DBS radio signals will generally have a much wider bandwidth than the signals of interest (both satellite and terrestrial), and thus the various components of DBS signals can be considered as being essentially at the same frequency.
There is a need for antennas or antenna systems that can receive radio frequency signals having circular polarization and/or linear vertical polarization. Furthermore, the antenna or antenna system should preferably be able to utilize different radiation patterns for each of these two functions. The antenna or antenna system should have a radiation pattern lobe with circular polarization directed towards the sky at the required elevation angle for satellite reception, and also have a radiation pattern lobe with linear polarization directed towards the horizon for terrestrial repeater reception.
Currently, there are antennas that can perform these two functions. One example of such an antenna is the quadrafilar helix antenna, which consists of four wires wound in a helical geometry. The drawback of this antenna is that it typically protrudes more than one-half wavelength from the surface of wherever it is mounted and, thus, if it is mounted on the exterior surface of a vehicle, it results in an unsightly and unaerodynamic vertical structure.
The antenna disclosed herein performs these two functions yet protrudes less than one-quarter wavelength from the roof of the vehicle. It is able to perform as a dual circular/linear polarized antenna with optimized antenna patterns for both the satellite and terrestrial links.
This invention offers a method of operating a spiral antenna simultaneously as a top-loaded monopole and in second resonance spiral mode.
The prior art includes:                (1) U.S. Pat. No. 5,313,216, “Multioctave Microstrip Antenna,” by Wang, et al. and assigned to Georgia Tech Research Corporation. This patent describes a micro-strip antenna that is between 0.02λc and 0.1λc, where λc is the wavelength at the geometric mean between the minimum and maximum operating frequencies above the ground plane. While this patent describes a spiral antenna mounted above the ground plane, it does not suggest dual mode operation or operation of the spiral as a top-loaded monopole.        (2) U.S. Pat. No. 4,051,477, “Wide Beam Microstrip Radiator,” L. R. Murphy, G. G. Sanford, and assigned to Ball Brothers Research Corporation. This patent describes a method of improving the low-angle radiation from an antenna by raising it above the ground plane on a pedestal.        (3) Nakano, et.al, “A Spiral Antenna Backed by a Conducting Plane Reflector,” IEEE Transactions on Antennas and Propagation, vol. 34, no. 6, pp. 791-796, June 1986.        (4) Wang, et.al, “Design of Multioctave Spiral-Mode Microstrip Antennas,” IEEE Transactions on Antennas and Propagation, vol. 39, no. 3, pp. 332-335, March 1991. This article provides more measured results for the spiral antenna configuration described in U.S. Pat. No. 5,313,216.        (5) Corzine, et.al, Four-Arm Spiral Antennas; Norwood, Mass.; Artech House; 1990. This book covers many aspects of four arm spiral antennas. The book documents many of the first advances in spiral antennas and feed networks.        (6) C. Balams, Antenna Theory Analysis and Design, 2nd edition, John Wiley and Sons, New York, 1997.        
Related art includes the following patent applications which are assigned to assignee of the present invention:                (1) D. F. Sievenpiper; H. P. Hsu; J. H. Schaffner; G. L. Tangonan, “An Antenna System for Communicating Simultaneously with a Satellite and a Terrestrial System,” U.S. patent application Ser. No. 09/905,795 filed Jul. 13, 2001, the disclosure of which is hereby incorporated herein by reference. An antenna system on a Hi-Z surface able to receive vertically and circularly polarized RF signals is disclosed by this application.        (2) D. F. Sievenpiper; J. H. Schaffner; H. P. Hsu; G. L. Tangonan, “A Method for Providing Increased Low-Angle Radiation in an Antenna,” U.S. patent application Ser. No. 09/905,796 filed Jul. 13, 2001, the disclosure of which is hereby incorporated herein by reference. A crossed slot antenna able to receive vertically and circularly polarized RF signals is disclosed by this application.        (3) D. F. Sievenpiper, “A Low-Profile Slot Antenna for Vehicular Communications and Methods of making and Designing Same,” U.S. patent application Ser. No. 09/829,192 filed Apr. 10, 2001, the disclosure of which is hereby incorporated herein by reference. A low-profile slot antenna able to receive vertically and circularly polarized RF signals is disclosed by this application.        